In a study performed by the present inventors, it was found that crystallization of an amorphous silicon film can be effected by heating the film at 550.degree. C. for a duration of about 4 hours by first depositing a trace amount of nickel or palladium, or other elements such as lead onto the surface of the amorphous silicon film and then heating it.
The elements above (catalyst elements capable of accelerating the crystallization) can be introduced by the use of plasma treatment or vapor deposition, or ion implantation. The plasma treatment is a method which comprises adding the catalyst elements into the amorphous silicon film by generating a plasma of an atmosphere such as nitrogen or hydrogen, using a material containing a catalyst element as an electrode(s) in a plasma CVD apparatus of a parallel plate type or of a positive column type.
However, it is not desirable to introduce a large amount of a catalyst elements since it hinders a reliability or electrical characteristics of a semiconductor device.
That is, the above mentioned element such as nickel which accelerate the crystallization (i.e. catalyst element) are necessary for the crystallization of the amorphous silicon film, but are preferably not incorporated in the crystallized silicon. These conflicting requirements can be accomplished by selecting, as the catalyst element, an element which tends to be inactive in crystalline silicon, and by incorporating the catalyst element in a minimum amount for the crystallization of the film. And for this purpose, the quantity of the catalyst element to be incorporated in the film needs to be controlled with high precision.
Also, in the case of using nickel as a catalyst element, a crystalline silicon film was fabricated by forming an amorphous silicon film and adding nickel through plasma treatment. The crystallization process and the like were studied in detail. The following findings were obtained as a result:
(1) In the case that nickel is introduced by plasma treatment onto an amorphous silicon film, nickel is found to penetrate into the amorphous silicon film to a considerable depth before the film is subjected to a heat treatment; PA1 (2) The initial nucleation occurs from the surface from which the nickel is incorporated; and PA1 (3) When a nickel layer is deposited on the amorphous silicon film by vapor deposition, the crystallization occurs in the same manner as in the case of effecting plasma treatment.
It can be concluded from the above findings that not all of nickel atoms incorporated by plasma treatment into the amorphous silicon film function effectively. That is, even if a large amount of nickel is introduced, some of the nickel does not function sufficiently. Accordingly, it can be considered that the point (surface) at which the nickel contacts silicon functions during the low temperature crystallization. Conclusively, nickel needs to be dispersed as finely as possible in the form of atoms. In other words, it is concluded that "nickel needs to be introduced in the vicinity of the surface of an amorphous silicon film in the form of atoms dispersed at a minimum concentration which is necessary for the low temperature crystallization.
A trace amount of nickel, i.e., a catalyst element capable of accelerating crystallization of an amorphous silicon, can be introduced only in the vicinity of a surface of the amorphous silicon film by, for example, vapor deposition. However, vapor deposition is disadvantageous in terms of the controllability of the film, and is therefore not suitable for precisely controlling the amount of the catalyst element.
The aforementioned problem can be overcome by applying a solution containing a catalyst element onto the surface of an amorphous silicon film in order that a predetermined quantity of the catalyst element is adsorbed by the surface of the amorphous silicon film and then effecting the crystallization by a heat treatment.
In accordance with this method, the concentration of the catalyst element incorporated in the amorphous silicon film can be controlled precisely by adjusting the concentration of the catalyst element in the solution and the period of time during which the solution is brought into contact with the amorphous silicon film.
The use of a solution for introducing a catalyst has various advantages, however, there is also a shortcoming resulting from the simplicity of the process. That is, the process is particularly sensitive to a contamination with particles. This is because when particles are present on the surface of the substrate to be coated, the surface cannot be brought into sufficient contact with the solution.
Furthermore, the process requires a step of heat treatment using a diffusion furnace and the like after the step of adding the catalyst element. In this step, the inside of the diffusion furnace must be prevented from being contaminated by the catalyst element. Also from this point of view, the step of applying the solution to the substrate must be carried out under strict control.